There has been a great demand for an optical communication system having large capacity and multi-functions, so that it is required in the system that an optical signal is generated to be propagated through optical paths and switched or changed over thereamong in a high speed. In order to obtain an optical signal, there has been realized a direct modulation method in which injection current of a semiconductor laser or a light-emitting diode is modulated method directly. In such a direct modulation, however, there are disadvantages in that high-speed modulation over several GHz is difficult due to effects of relaxation oscillation, it is difficult to realize a coherent optical transmission due to wavelength changes, etc. Therefore, an outer modulation method has been developed in which an optical signal is modulated by an outer optical modulator. In such an optical modulator, a waveguide-type optical modulator including optical waveguides formed in an electro-optical crystal substrate has advantages such as a compact size, a high efficiency and a high-speed operation.
On the other hand, an optical switch is used as means for changing an optical path of an optical signal. A mechanical optical switch has been used in which a prism, a mirror or an optical fiber is moved mechanically to change an optical path of an optical signal. Such a mechanical optical switch has disadvantages of a low-speed operation and a large size which is unable to be adopted for a matrix system. A waveguide-type optical switch has been also developed because of a high-speed operation, a high integration ability, and a high reliability, etc. In such a waveguide-type optical switch, a waveguide-type optical switch using a ferromagnetic material such as lithium niobium oxide (LiNbO.sub.3) crystal has several advantages such as low loss because of low optical absorption, a high efficiency because of large electro-optical effects, etc. As a waveguide-type optical switch, an optical control device such as an optical modulator or switch including a directional coupler, a Mach Zehnder-type optical modulator, a balance-bridge type optical modulator or switch, a total reflection-type optical switch has been reported. An optical switch including a directional coupler using a Z-plane LiNbO.sub.3 substrate can switch an optical path of an optical signal in any polarizing plane. Such an operation is known as a polarization-independence operation.
The polarization-independence operation can be realized by matching the switching voltage of TE and TM polarized lights which are vertical to each other in an optical signal having a random polarization plane. In other words, applied voltage for obtaining bar-state in a directional coupler in any plane of polarization is the same value.
A conventional optical switch includes an electro-optical crystal substrate consisting of Z-plane LiNbO.sub.3 substrate, first and second waveguides of a predetermined length parallel to each other formed in a portion of the substrate in the vicinity of the surface thereof, a buffer layer formed on the substrate covering the surface thereof located the area where the waveguides are formed, and first and second electrodes formed on the buffer layer locating above the first and second waveguides, respectively. In the optical switch, the two waveguides are designed to have the same propagation constant.
In operation, optical signals propagate through the first and second waveguides. An optical signal which propagates through each of the first and second waveguides changes its propagation path to the other waveguide and optical energy is transferred from one waveguide to the other if the directional coupler has the predetermined length. Such a state is known as a cross-state. On the other hand, the optical signal propagates through one waveguide and does not change the path on a predetermined condition. Such a state is known as a bar-state. Such states should be realized independent on polarization of optical signals.
In the cross-state, the complete coupling length of the directional coupler is determined to be the same length between TM and TE polarizations, and there is applied no voltage between the first and second electrodes to generate a Z-axial field, so that 100% of optical energy is transferred from one waveguide to the other. On the other hand, in the bar-state, a voltage is applied between the first and second electrodes, so that the propagation constant becomes different in the first and second waveguides which should have the same propagation constant when there is no voltage applied therebetween.
According to the conventional optical control device, however, there is a disadvantage in that a phase mismatching occurs and the transfer of optical energy becomes difficult, so that the optical signal propagates through one waveguide without changing the path. The switching voltage is defined as a voltage where the bar-state occurs. In addition, the operation voltage for switching or modulating both TM and TE polarized lights simultaneously is required to be high. Therefore, it is difficult to obtain a high cross-talk characteristic, a high distinction ratio, or a low operation voltage.